1. Field of the Invention
The present invention relates to an amplifier apparatus of the type used as an output stage of a driver circuit, for example to drive a semiconductor laser device. The present invention also relates to a control circuit and/or a driver circuit comprising the amplifier apparatus.
2. Description of the Prior Art
It is known to employ a differential pair configuration in an output stage of a driver circuit for a laser device. The differential pair configuration typically comprises a first transistor and a second transistor independently coupled to a first supply rail by respective collector terminals of the first and second transistors. An emitter terminal of the first transistor is coupled to an emitter terminal of the second transistor, both emitter terminals of the first and second transistors also being coupled to a second supply rail via a current source. A potential difference of at least about 0V needs to exist between the collector terminal and the base terminal of the first or second transistors, depending upon which of the first or second transistors is conducting. If the potential difference is less than 0V, i.e. if the collector terminal of the first or second transistors is at a lower potential than the base terminal thereof, the first or second transistor will saturate resulting in a reduction in the switching speed of the first or second transistor. Given that at least approximately 400 mV is typically dropped across the current source and about 0.9V is likely to be dropped across the base-emitter junctions of the first or second transistors when conducting, the potential difference between the collector terminal of the first or second transistor, when conducting, and the second supply rail is likely to be greater than 1.2V. Consequently, the voltage headroom between the first supply rail and the collector terminal of the first or second transistor, when conducting, is limited.
In order to provide increased headroom for the differential pair configuration, the driver circuit can be operated at a higher operating voltage. However, the higher operating voltage is undesirable, because other components of the driver circuit, for example higher speed Complimentary Metal Oxide Semiconductor (CMOS) Integrated Circuits (ICs), will not operate at the higher operating voltage.
An alternative to using the differential pair configuration above, is to employ a single transistor arranged in a common emitter (open collector) configuration. The single transistor arranged in the common emitter configuration (hereinafter referred to as a xe2x80x9ccommon emitter transistor arrangementxe2x80x9d) comprises a single bipolar transistor, a collector terminal of the single transistor being coupled to the first supply rail via a first load. An emitter terminal of the single transistor is coupled to the second supply rail via, for example, a second resistor and the base terminal of the single transistor receives a single input signal corresponding to a data signal. A single output signal is present at the collector terminal of the single transistor. The common emitter transistor arrangement requires less voltage headroom than the differential pair configuration, but switches at a slower speed than the differential pair configuration and the current amplitude of the single output signal is difficult to control. Additionally, the current of the single output signal is not balanced by an opposing current, resulting in occasional large current surges into the second supply rail (known as xe2x80x9cground bouncexe2x80x9d). The large current surges into the second supply rail cause Electromagnetic Interference (EMI) spikes, which are undesirable. Also, it is difficult to control DC content of the single output signal.
AC coupling the output of the common emitter transistor arrangement can obviate the lack of voltage headroom but does not work without an internal DC load; the internal DC load dissipates power. Also, the AC coupling removes frequency component from the single output signal below a given frequency, depending upon values of electrical components used to implement the AC coupling. Removal of the low frequency components can, in some cases, cause distortion of a waveform constituting the single output signal.
According to a first aspect of the present invention, there is provided an amplifier circuit apparatus comprising a first input and a second input for respectively applying a first input signal and a second input signal, the first input being arranged to control a first active device of a first common emitter circuit having a first output, characterised by first circuit means coupled to the first common emitter circuit and the second input so as to enable, when in use, the first common emitter circuit to generate an output signal at the first output, the output signal corresponding to an amplification of a difference between the first input signal and the second input signal by a differential gain.
Preferably, the amplifier circuit is powered by a low voltage supply, for example below 3.6V, such as below 3.3V. Most preferably, the low voltage supply is below 2.5V.
Preferably, the apparatus further comprises a second active device of a second common emitter circuit coupled to the second input and having a second output, the first circuit means being arranged to enable, when in use, the first and second common emitter circuits to generate the output signal between the first and second outputs, the output signal corresponding to an amplification of a difference between the first input signal and the second input signal by a differential gain.
Preferably, the first circuit means comprises a third input to control a third active device and a fourth input to control a fourth active device, the first and second inputs being respectively coupled to the third and fourth inputs, and the third and fourth active devices being cross coupled.
Preferably, the apparatus further comprises at least one further circuit means arranged to mirror a predetermined amount of current flowing through the first and/or second common emitter circuits so as to provide at least one predetermined function.
Preferably, the predetermined function is the generation of a signal indicative of the output signal for controlling the output signal. More preferably, the at least one further circuit means comprises second circuit means comprising a fifth active device arranged to generate a first feedback component signal indicative of a first current flowing through the first active device.
When the first circuit means comprises the second active device, the second circuit means preferably comprises a sixth active device arranged to generate a second feedback component signal indicative of a second current flowing through the second active device.
Preferably, an amplitude of a third current flowing through the fifth active device is less than an amplitude of a first current flowing through the first active device.
Preferably, an amplitude of a fourth current flowing through the sixth active device is less than an amplitude of the second current.
Preferably, the amplitude of the third current is proportional to the amplitude of the first current.
Preferably, the density of the fourth current is proportional to the amplitude of the second current.
Preferably, the at least one predetermined function is a prevention of the output signal comprising a current level that exceeds a predetermined current level.
Preferably, the at least one further circuit means comprises third circuit means comprising a seventh active device arranged as a first integrated diode.
When the first circuit means comprises the second active device, the third circuit means preferably comprises an eighth active device arranged as a second integrated diode.
Preferably, an amplitude of a first current flowing through the first active device is proportional to a second amplitude of a fifth current flowing through the seventh active device.
Preferably, an amplitude of a second current flowing through the first active device is proportional to a an amplitude of a sixth current flowing through the eighth active device.
Preferably, the amplitude of the fifth current is lower than the amplitude of the first current.
Preferably, the amplitude of the sixth current is lower than the amplitude of the second current.
According to a second aspect of the present invention, there is provided a driver circuit for a laser device comprising the amplifier circuit apparatus as set forth above in relation to the first aspect of the present invention.
According to a third aspect of the present invention, there is provided an optical communications network comprising the amplifier circuit apparatus as set forth above in relation to the first aspect of the present invention.
It is thus possible to provide an amplifier circuit apparatus capable of providing the advantages of the single common emitter transistor arrangement without the disadvantages of slower switching speeds and difficult current amplitude control associated with the single output signal. The amplifier circuit apparatus can thus generate a balanced differential output signal that uses less voltage headroom than conventional differential-pair amplifier circuits, and does not result in ground bounce and, hence, EMI spikes. Also, it is possible to control DC content of the differential output signal. Additionally, current flowing through the laser device can be limited, thereby ensuring that light emissions from the laser device are maintained within safe parameters.